For LED lighting applications, down-converting nanoparticles such as cadmium selenide quantum dots, indium phosphide quantum dots, and lead-halide perovskite quantum dots may offer several significant technological advantages over conventional inorganic phosphors. Of primary interest are their narrow photoluminescence emission line widths, which may be below 40 nm FWHM (full width at half maximum), along with their tunable photoluminescence peak position. Both properties are typically unavailable in conventional down-converting phosphors, such as cerium(III)-doped yttrium aluminum garnet (YAG:Ce3+), used for LED lighting and display applications.
A major obstacle to the commercial deployment of down-converting nanoparticles in lighting products, however, is their stability. Products for general lighting are routinely offered with operational lifetimes greater than 25,000, 50,000, and even 100,000 hours of use. Accordingly, the demands for material stability and product reliability are exceedingly high. To date, quantum dots of the types identified above have generally lacked the stability required to be commercially viable under typical LED package operating conditions.
A common cause of the instability of luminescent nanoparticles is their sensitivity to atmospheric oxygen and moisture. Under the harsh operating conditions of a LED, quantum dots may react with atmospheric oxygen and moisture, which may lead to an unacceptable loss in luminous flux output or change in color point.